In an electronic device whose battery cannot be exchanged by a user, the battery itself is implemented on a circuit board of the electronic device as one electronic component. An electronic device handling a large current requires a safety device to cut off the current in the case of increase in temperature of the battery due to, for example, short-circuit at the inside and outside of the battery. In view of this, some batteries capable of large current discharge such as a lithium battery incorporate a Positive Temperature Coefficient (PTC) element. As is well-known, while usually having a low resistance, the PTC element is a chip component that has a laminated structure with metal foils on both surfaces of plate-shaped high-polymer material (polymer) and ceramic material (hereinafter also referred to as a PTC material) whose electrical resistance increase as the temperature increases. The PTC element, which is incorporated in the battery, is disposed in a sealing body doubling as an electrode terminal of one side of the battery.
However, some batteries include a sealing structure into which the PTC element cannot be built. Alternatively, there may be a case where current cut-off characteristics of the PTC element incorporated in the battery does not match characteristics required for the electronic device. Accordingly, as an electronic component to handle such case, a PTC element externally attached to the electrode terminal of the battery (hereinafter also referred to as an externally-attached PTC element) is available. The battery with the externally-attached PTC element attached is incorporated in the electronic device.
FIG. 1 illustrates a conventional externally-attached PTC element 101 attached to a cylindrical battery 110. FIG. 1 hatches members constituting the externally-attached PTC element 101 differently. As illustrated in FIG. 1, the externally-attached PTC element 101 has a structure of laminating metal plates (120, 130) on respective front and back of a PTC element 10 as a plate-shaped chip component and is attached to an electrode terminal 111 on one end surface of the cylindrical battery 110.
For example, fixedly securing the one metal plate 130, which constitutes the externally-attached PTC element 101, to the electrode terminal 111 on the one side of the cylindrical battery 110 by spot welding or a similar method attaches the externally-attached PTC element 101 to the cylindrical battery 110. To implement the cylindrical battery 110 with the externally-attached PTC element 101 attached on, for example, the circuit board, a lead wire 50 is connected to the other metal plate 120 of the externally-attached PTC element 101 using, for example, a solder 40.
A terminal plate 160 made of, for example, only a metal plate is attached to an electrode terminal 112, which is on the other end surface of the cylindrical battery 110. Obviously, the lead wire 50 is soldered to the terminal plate 160. Soldering the lead wire 50 to the circuit board implements the cylindrical battery 110 on the circuit board.
In addition to the externally-attached PTC element 101 illustrated as the example in FIG. 1, as illustrated in FIG. 2A and FIG. 2B as examples, there is provided an externally-attached PTC element 201 that sandwiches the PTC element 10 by two strip-shaped metal plates (220, 230).
FIG. 2A is a drawing illustrating a stacked structure of this externally-attached PTC element 201. FIG. 2B is a drawing illustrating the cylindrical battery 110 with the externally-attached PTC element 201 attached. The externally-attached PTC element 201 illustrated as the example in FIG. 2A is formed of the two electrode plates (220, 230), which are both made of the strip-shaped metal plate, opposed via the PTC element 10. While base end sides (222, 232) of both electrode plates (220, 230) are opposed to one another, distal end sides (221, 231) of the respective electrode plates (220, 230) extend so as to face directions opposite to one another. The PTC element 10 is disposed in an opposed area where both electrode plates (220, 230) are stacked to one another.
Mounting the externally-attached PTC element 201 illustrated in FIG. 2A to the cylindrical battery 110 brings the distal end side 231 of the one metal plate 230 in contact with the electrode terminal 111. Since the two strip-shaped metal plates (220, 230) extend in the identical direction through the opposed area, the externally-attached PTC element 201 significantly projects to the outside of the cylindrical battery 110. Implementing the cylindrical battery 110 in this state on the circuit board significantly separates the cylindrical battery 110 itself from the circuit board, thereby inhibiting downsizing of the electronic device.
As illustrated in FIG. 2B, the metal plate 230 on the side in contact with the electrode terminal 111 is bent along the outer shape of the cylindrical battery 110 in the middle of the extension in some cases such that the opposed area of the two electrode plates (220, 230) becomes parallel to the side surface of the cylindrical battery 110.
The following Patent Literature 1 describes an externally-attached PTC element with a structure of sandwiching a plate-shaped PTC element by two strip-shaped metal plates. The following non-Patent Literature 1 describes a structure, properties, or a similar feature of a PTC element referred to as so-called “PolySwitch” using a polymer as the PTC material.